All-optical processing in communications systems

ABSTRACT

An all-optical processing system coverts or interfaces optical signals from a wavelength division multiplexed (WDM) form to an optical time divisional multiplexed (OTDM) form. The initial WDM signal typically comprises a non-return to zero (NRZ) signalling format. The system includes a plurality of NRZ data modulated, cw optical WDM channels which are cross-phase modulated, and thus are spectrally broadened, in an optical non-linear element, by a strong clock pulse signal. The resultant signal comprises an RZ representation of the original NRZ signal. The RZ signal is temporally shifted by a dispersive element which temporally shifts each wavelength channel by a predetermined amount, to produce a wavelength-interleaved OTDM signal. The signal is then wavelength converted by cross-phase modulation with a cw control beam in a second non-linear optical element to provide a single wavelength OTDM signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical processing apparatus and systems, andto methods of processing optical communication signals, and particularlyto systems, methods and apparatus for manipulating wavelength divisionmultiplexed optical signals.

2. Related Art

Optical fibres are an extremely efficient transmission medium.Presently, the capacity of optical fibre communications systems islimited by certain factors, including the way in which the optical fibrebandwidth is utilised, and the opto-electronic components required tocontrol certain optical communication processing functions.

The first factor, that of bandwidth use, is generally addressed by theuse of various multiplexing techniques, for example wavelength divisionmultiplexing (WDM) or optical time division multiplexing (OTDM).

The second factor has been extensively investigated over the past six orseven years, the results being demonstrations of all-optical processingfunctions in optical fibres and semiconductor optical devices. Anoptical fibre communications network incorporating only all-opticalprocessing functions would potentially provide communications capacityfar beyond that which is currently available in optical fibrecommunications networks incorporating very much slower opto-electronicprocessing functions.

In terms of bandwidth usage, WDM networks have received considerableattention in recent years, and are likely to provide optical routing in,for example, a metropolitan or national network, where a large nodedensity makes the simple passive demultiplexing (wavelength filtering)associated with WDM attractive. However, the combination of dispersionand fibre non-linearity potentially restricts the size of WDM networks,or the ability to expand WDM networks, if traditional signalling formatsare employed. Therefore, presently OTDM is more likely to findapplication over wider geographical areas, with a smaller number ofhigher capacity optical switches, since a single wavelength, multiplexedchannel system such as OTDM is not so susceptible to the detrimentaleffects non-linearity and dispersion as a WDM system, particularly whensoliton transmission effects are employed to balance non-linearityagainst dispersion. Furthermore, gain flatness equalisation orpre-emphasis techniques are not an important consideration for singlewavelength OTDM systems, whereas such techniques would be an importantaspect of the design of a corresponding WDM system, considerablysimplifying amplifier (or power) management.

Recognising the problems of scalability associated with WDMcommunications networks, but at the same time appreciating that WDM hasmany advantages, for example simple passive demultiplexing, theapplicants have considered that in future there might be a need for anall-optical communications network which is potentially able to dealwith WDM traffic (eg on a local scale), OTDM traffic (eg oninternational trunk routes), and soliton traffic (eg on informationsuper-highways). To be effective, such an optical network would alsoneed to be able to convert between any two of the traffic formatsemployed, otherwise universal interconnection to, and informationinterchange across, the network would be restricted.

Presently, generation and transmission of WDM, OTDM and soliton opticalsignalling formats is known and has been widely reported. Also, Lacey,et al. "All optical WDM to TDM transmultiplexer", Electronics Letters,Sep. 15, 1994, pp 1612-1613, proposes WDM to TDM conversion firstly bysplitting the WDM signal into its constituent channels using wavelengthselective filters and mixing each channel with a clock pulse in separaterespective optical amplifiers. This has the effect that gain compressioncauses wavelength conversion and reduces the width of the WDM datapulses. Then, each channel is delayed by separate respective opticaldelay lines having different delays, and finally all the channels areremultiplexed using an optical coupler. Bigo, et al, "Bit-rateenhancement through optical NRZ-to-RZ conversion and passivetime-division multiplexing for soliton transmission systems",Electronics Letters 1994, vol. 30, pp 984-985, proposes using an opticalloop mirror as an AND gate for an NRZ data signal and a clock signal toprovide NRZ to RZ conversion. Further, Bigo et al. proposes multiplexinga plurality of these AND outputs to provide bit-rate enhancement (TDM).

Throughout the present description, the terms "square" and "pulsed",with respect to wave forms, are intended to be synonymous andinterchangeable with "NRZ" and "RZ" respectively.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides apparatusfor processing a wavelength division multiplexed optical signal,comprising:

input means to receive a first optical signal and a second opticalsignal, said first optical signal comprising at least two data channelsof different wavelength and said second optical signal being of a singlewavelength and comprising a stream of pulses, the pulses having a pulserepetition rate at least as high as the bit rate of the highest datarate data channel;

a non-linear optical element having input means to receive said firstand second optical signals and output means to provide a third opticalsignal, the third optical signal being representative of a logical ANDfunction of the first and second optical signals and comprising pulseseach having wavelength components corresponding to the respective datachannels; and

means to apply a wavelength-dependent delay to each wavelength componentof the third optical signal to provide a fourth optical signal.

This apparatus forms the first part of an overall apparatus for carryingout NRZ to RZ conversion.

By correctly placing the apparatus, for example, in a metropolitan areanetwork (MAN), standard WDM NRZ signalling can be employed for alltraffic within the MAN, whilst long-haul traffic, for example to asecond, far-removed MAN can be routed across a soliton supportingcommunications link, by converting the NRZ format signal to a RZ formatin accordance with the present invention, and subsequently to a solitonformat signal by a known method. This approach has the advantage thatall sources and wavelength converters/switches within each MAN need notnecessarily be soliton compatible. Thus, advantageously, existing WDMinfrastructure can be maintained, minimising the capital cost ofinstalling the system.

The non-linear optical element preferably comprises a travelling wavesemiconductor laser amplifier (TWSLA). In a TWSLA, a strong clock pulsesignal is provided, having a pulse repetition rate at least as high asthe highest bit-rate WDM channel, which periodically modulates the TWSLAcarrier density, imposing cross-phase modulation (XPM) on WDM channelspropagating through the amplifier. The result of the XPM is aspectrally-broadened WDM signal wherever the WDM signal is coincidentwith the clock pulse signal.

When the optical signals are spectrally broadened by XPM in a non-linearelement, the apparatus preferably further comprises a cyclic filterdownstream of the non-linear element to perform a frequencydiscrimination function. The cyclic filter eliminates the un-modulatedbackground of the WDM channels so it should have regular pass and stopbands, with a good stop band extinction. Suitable filters include MachZehnder interferometers, Fabry Perot interferometers and birefringentfilters.

One form of suitable cyclic filter is a birefringent fibre incorporatinginput and output polarisation control. The filter selects the requiredwavelengths by modifying the polarisations of each WDM channel in thefibre, so that the required wavelengths coincide with an outputpolariser. An arrangement of this type can provide excellent extinctionwith relatively broad pass-bands. The resulting filtered signal issubstantially a WDM RZ, pulse signal representation of the WDM NRZsignal.

A significant advantage of incorporating an non-linear optical elementinto the apparatus of the present invention is that conversion from NRZto RZ of all WDM channels is carried out simultaneously in a singleelement, and that the operating speed is limited by opticalnon-linearity rather than by the speed of alternative opto-electronicdevices.

A significant advantage of using XPM in a TWSLA as the non-linearelement is the intrinsic wavelength insensitivity of the TWSLA, andwhilst similar XPM effects have been demonstrated in optical fibre,constraints imposed by chromatic dispersion in optical fibres precludemulti-channel operation over a wide wavelength range, although the useof dispersion-flattened fibre may alleviate some of these difficulties.

An alternative non-linear element to a TWSLA is a non-linear opticalloop mirror (NOLM). In particular, a NOLM incorporating a semiconductorlaser amplifier is preferred since fibre NOLMs may suffer from unwanteddispersion effects and phase matching problems.

A NOLM has an advantage over the method described above that a cyclicfilter is not required since the cross-phase modulated signal portionsare switched out of a different output from the non-cross-phasemodulated signal portions, rather than being combined.

Other non-linear elements which could be used in place of a TWSLA or aNOLM include a polarisation rotation gate, a non-linear Mach Zehnderinterference gate, a non-linear directional coupler, or a non-linearFabry Perot interferometer. This list of alternatives is by no meansexhaustive, and does not limit the choice to those alternatives listed.A further alternative, limited in processing speed, is electro-opticmodulation using for example electro-absorption modulators to form thepulses.

In a preferred embodiment, the present invention further comprises shiftmeans for temporally shifting each one of the discrete wavelengthchannels by a different predetermined amount to provide awavelength-interleaved time division multiplexed (WITDM) signal.

In this way, a WDM signal (RZ format), is converted to a pseudo-OTDMsignal which has the temporal form of an OTDM signal, but not thespectral form (that is, at this stage the signal comprises a series ofpulses which sequentially cycle through the different wavelengths of theWDM signal). The formation of a WITDM signal can be an intermediate steptowards conversion to an OTDM signal, as described below.

The shift means preferably comprises a dispersive element, whichprovides a fixed amount of chromatic dispersion to delay each one of thediscrete wavelength channels by a different amount; thus producing theWITDM signal.

The dispersive element could comprise a suitable length of standardoptical fibre, however a pair of refraction gratings or other dispersiveelements could be used instead. However, using a length of standardoptical fibre has the advantage that the entire operation of convertingfrom a purely wavelength multiplexed signal to a WITDM signal isachieved in a single optical fibre path. In contrast, in known systems,conversion from WDM to a time division multiplexed format would involvedelaying individual channels each by a different amount using differentdelay lines, and then combining the delayed signals in an opticalinterleaver. Such systems may be unreliable since optical interleaverstypically exhibit poor stability. There is also the greater cost of suchsystems to consider, compared to the cost of a length of standardoptical fibre which can achieve the same effect. The length of opticalfibre required is determined by the extent of temporal shifting requiredbetween WDM channels, and by the dispersion constant of the opticalfibre.

The effect of the dispersion on the pulses in an optical fibre should beconsidered, and it would appear that lower dispersions would be requiredto reduce any unwanted temporal broadening, resulting in broad RZpulses. It is perhaps fortunate, however, that the chirp experienced bypulses formed in a TWSLA can produce pulse compression in standardoptical fibre in some circumstances. Thus, preferably, an optical fibredispersive element both compresses and interleaves the pulses in apreferred scheme.

It will be appreciated that the use of dispersion, for example in alength of optical fibre, as described above, to provide a WITDM signalfrom a RZ format WDM signal, is not limited to use in accordance withthe present invention. Indeed, a WDM signal (RZ format) may be providedby any source and the subsequently formed WITDM signal could be used inaccordance with the present invention, or in any other apparatusrequiring such a signal.

In a preferred embodiment, the present invention further comprises asecond non-linear optical element for converting the WITDM signal to asingle wavelength OTDM signal.

This second non-linear optical element can conveniently embody thefeatures of the first non-linear optical element described above.

Therefore, preferably the second non-linear optical element comprises asecond TWSLA which operates in a similar fashion to the operation of thefirst TWSLA, described above. However, in this case, the WITDM signal isarranged to phase-modulate a cw beam, and the cyclic filter is replacedby a bandwidth limiting filter, to isolate the resultant, singlewavelength, OTDM signal.

Alternatively, the non-linear element can be any other suitablenon-linear element, for example a NOLM or a Mach Zehnder Interferometeretc, as before.

In a still further embodiment, the speed of operation of a TWSLA,embodied in the second non-linear stage, can be increased by operatingthe amplifier in accordance with our co-pending European patentapplication number 93308066.5 (filed on Oct. 11, 1993), in which athird, pump, beam is injected into the TWSLA to suppress data patterningeffects that may otherwise occur due to temporally non-uniform carrierdensity dynamics. The use of such a pump beam to pin the Fermi level andsuppress data patterning, is described in further detail in theapplication mentioned, the disclosure of which is incorporated herein byreference.

An advantage of the present invention is that extremely spectrally puredata pulses are produced, without using a high specification pulsesource. Normally, a pulse source used for producing OTDM signalsrequires both spectral purity and temporal stability, and hence isrelatively expensive. However, in an apparatus according to the presentinvention, the clock pulse source needs only to provide pulses which aretemporally stable, since the spectral purity is achieved through thesecond non-linear element and filter (if necessary) arrangement.Therefore, the light source for the OTDM system can be a relativelycheap DFB laser. Alternatively, if a high speed pulse source isavailable, a second converter may be used to modulate the pulse stream.

A further advantage of the present invention is that all the activecomponents can be semiconductor components, for example TWSLAs, whichallows scope for integration (although development of an on-chip cyclicfilter and a dispersive element would be required).

According to further aspects, the present invention also providesmethods and systems as described in more detail in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only,which reference to the accompanying drawings, of which:

FIG. 1 is an experimental system to demonstrate conversion of a fourchannel WDM signal carrying NRZ data to a WDM signal carrying RZ data;

FIG. 2 shows a polarisation filter arrangement used in the system ofFIG. 1;

FIG. 3 shows the stages of spectral broadening for a single NRZ pulse;

FIG. 4 shows spectral transformations for four RZ wavelength channels;

FIGS. 5A-5D show eye diagrams for the four RZ pulse wave forms;

FIG. 6 shows BER measurement comparisons for the system in FIG. 1;

FIG. 7 is system representation suitable for full WDM to OTDM signalconversion;

FIGS. 8A to 8G are idealised graphical representations of the signalspectra at points in the system of FIG. 7; and

FIGS. 9A to 9G are idealised graphical representations of thetime-varying amplitude signal wave forms at points in the system of FIG.7.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The system of FIG. 1 is used to generate a four-channel NRZ format, WDMoptical signal, to demonstrate conversion of the signal to acorresponding RZ format signal. In the system of FIG. 1, a clock signalcomprising equally spaced optical pulses is generated by an externalcavity semiconductor laser 110 (1545 nm) mode-locked at 10 GHz, with apulse width of 13 ps (assuming a Gaussian pulse shape) and atime-bandwidth product of ˜0.45.

Four cw channels (1554.2 nm, 1556 nm, 1557 nm, 1558.4 nm) are generatedby four cw DFB laser diodes 100. The channels are combined by a 4-to-1fibre coupler 105 and, for the purposes of demonstration, are modulatedwith a single 10 Gbit/s 2⁷ -1 PRBS using a lithium niobate (LiNbO₃)intensity modulator 130, driven by a suitable pattern generator 160.

The clock and NRZ format data signals are amplified in opticalamplifiers 120 and 122, for example erbium doped optical fibreamplifiers, and are fed into a TWSLA 145 through a WDM coupler 140. Theclock signal is amplified to around 12 dBm, and the NRZ signal isamplified to between 1 to 3 dBm. The TWSLA 145 is a bulk device, with acoupling loss of ˜6 dB per facet. Although a TWSLA provides XPM, anyelement providing a similar function would be suitable, in this case.

A cyclic filter 150 (described in more detail below, with reference toFIG. 2) which is implemented using polarisation rotation in abirefringent fibre, performs a frequency discrimination functiondownstream of the TWSLA 145 to process all four channels simultaneously.The filter cycle is ˜0.7 nm, and the extinction ratio is about 30 dB.Essentially, the filter 150 removes the unperturbed parts of the datasignals (the background components) passing signals that are coincidentwith, and therefore chirped by, the clock signal.

Finally, a 0.5 nm tuneable band pass filter 154 selects one of the fourRZ channels, for bit-error-rate (BER) measurement purposes. An errordetector 158 compares signals selected by the band pass filter 154 andreceived by an optical receiver 156, with copies of the signals from thepattern generator 160.

The results of the BER measurements for each of the channels in turn aredescribed below with reference to FIG. 6.

With reference to FIG. 2, the cyclic filter 150 comprises an input 200to a combination of a polarisation controller (PC) 210 and a polariser220. The PC 210 and the polariser 220 are tuned to ensure that anoptical signal entering the filter has a well-defined polarisation. ThePC 210 is included to enable fine adjustment of the input polarisation.

An optical amplifier 230, downstream of the polariser 220, is includedto compensate for signal losses due to the initial polarisationselection stage. Any form of optical amplifier can be used, although asuitable length of erbium-doped optical fibre is preferred.

The optical signal, having passed through the polarisation selectionstage, has a well-defined polarisation. In the filter 150, the periodicspectral shifts imposed on the reference beam by the data pulses in theTWSLA 145 experience a wavelength-dependent polarisation rotation in abirefringent element 250 positioned downstream of the amplifier 230. Thebirefringent element 250 in this case is a birefringent fibre which hasa length of 100 m and a polarisation mode dispersion of 10 ps. Byoptimising a polarisation controller 260, which is positioned downstreamof the birefringent fibre 250, the spectrally un-shifted component ofthe wave form is blocked by a polariser 270 positioned downstream of thepolarisation controller 260, resulting in the transmission of 10% of thelight incident on that polariser. The 10% of the light consists of atrain of wavelength-converted pulses, where the pulses effectivelycorrespond to a RZ representation of the original signal.

Other known types of wavelength filtering arrangement can easily replacethe cyclic filter arrangement described, for example Mach Zehnderinterferometers or Fabry Perot interferometers.

FIG. 3 shows the spectral broadening stages of one selected NRZ channel.Trace A represents the un-broadened NRZ channel, trace B represents theNRZ channel spectrally broadened by a clock pulse, and trace C shows thecw component removed by the cyclic filter.

In FIG. 4, the spectral peaks at the DFB wavelengths (D_(x)), with andwithout the clock wave form, are the unconverted residual NRZ signalswhich are removed by the cyclic filter when its stop bands are alignedwith the peaks.

FIGS. 3 and 4 show that the spectral broadening is asymmetric, beingbiased to the longer wavelength side due to the gain recovery mechanismin the TWSLA 145. Because of the asymmetry in spectrum, it is possibleto remove the un-broadened parts with only ˜5 dB insertion loss for thebroadened signals. The alignment of the four wavelengths with respect tothe filter pass bands is realised by a fine tuning of both thetemperature and bias current of the DFBs 100.

FIGS. 5A-5D show eye diagrams for all four wavelength channels. As canbe seen, the diagrams do not show patterning effects, which arecompletely suppressed by the strong clock signal. The pulse widths ofthe converted RZ signals are ˜15 ps (assuming Gaussian pulse shape)which is very close to the clock pulse width of ˜13 ps.

To demonstrate the low noise characteristics of the converted RZsignals, BER measurements are carried out for all four channels, theresults of which are shown in FIG. 6. The receiver 156 sensitivity(@BER=10⁻⁹) spread among the 4 channels is ˜1.5 dB, and there is nonoticeable error floor at BER=10⁻¹¹, illustrating the excellentperformance of the scheme.

FIG. 7 illustrates a system suitable for converting four NRZ opticalsignals at different wavelengths into a single OTDM channel.

In the system, a clock signal comprising equally spaced optical pulsesis generated by an external cavity semiconductor laser 710 (1545 nm)mode-locked at 10 GHz, with a pulse width of 13 ps (assuming Gaussianpulse rate) and a time-bandwidth product of ˜0.45.

Four WDM format NRZ channels (1554.2 nm, 1556 nm, 1557 nm, 1558.4 nm)are generated by four cw DFB laser diodes 700, which produce beams whichare modulated by individual modulators 730 (to model four separatewavelength channels). The four channels are combined by a 4 to 1 WDMfibre coupler 705. The combined signal is then preferably passed into anoptical element 708 which simulates the effects of an opticalcommunications link, for example by dispersing and attenuating thesignal. In practice, a link may be thousands of kilometres long, and mayinclude one or more stages of amplification, signal regeneration, and/orswitching. Thus, the parameters of the element 708 may be varied tosimulate different link parameters. The element 708 may comprisecombinations of different types of optical fibre, dispersive elementsand maybe loop mirrors, however, the actual arrangement depends on theeffects to be simulated. For the purposes of demonstration, opticalelement 708 may be omitted from the system altogether.

The clock and NRZ format WDM data signals are amplified in erbium dopedoptical fibre amplifiers 720 and 722 which provide the necessary signallevels for subsequent stages: the clock signal is amplified to around 12dBm and the NRZ format signal between 1 to 3 dBm. The signals are thenfed into a TWSLA 745 through a WDM coupler 740. The TWSLA 745 is a bulkdevice, with a coupling loss of ˜6 dB per facet.

A cyclic filter 750 (described above, with reference to FIG. 2), whichis implemented using polarisation rotation in a birefringent fibre,performs a frequency discrimination function downstream of the TWSLA 745to process all four channels simultaneously. The filter cycle is ˜0.7nm, and the extinction ratio is about 30 dB. Essentially, the filter 750removes the unperturbed part of the data signals (the backgroundcomponent) passing signals that are coincident with, and thereforechirped by, the clock signal.

The output of the cyclic filter 750 is dispersed by a dispersive element760, this element being a length of standard optical fibre. For WDMchannels 1.4 nm apart, 25 ps delay between adjacent channels requires17.8 ps/nm of dispersion, which is provided by around 1 km of standardoptical fibre. Alternatively, any length of suitably dispersive opticalfibre, which provides the required amount of dispersion, could be used.

The dispersed signal is amplified by an amplifier 726 and is fed into asecond TWSLA 770, through a WDM coupler 765, along with a cw referencesignal amplified by an amplifier 724 and originating from an opticallyamplified DFB semiconductor laser 772. The WDM pulse stream is amplifiedto a mean optical power of +12 dBm and the co-propagating cw referencesignal is amplified to a mean optical power of 4.9 dBm.

Optionally, a separate optically amplified counter-propagating (orco-propagating) cw DFB pump beam is injected into the second TWSLA 770,from a DFB semiconductor laser 775, to suppress the data patterningeffects that might otherwise occur due to temporally non-uniform carrierdensity dynamics.

The second TWSLA 770 is a polarisation insensitive bulk-layer devicewith a peak wavelength of 1.54 μm, operating with a DC bias current of200 mA. A filter element 780 positioned downstream of the TWSLA 770,comprises a tuneable band pass filter and polariser which is used toeliminate the residual amplified spontaneous emission and to define thepolarisation of the reference signal at the output of the TWSLA 770. Thefilter 780 is typically similar to the cyclic filter 150 described abovewith reference to FIG. 2. However, for this filter, emphasis is placedon a single accurate pass band and strong rejection about the requiredoutput signal wavelength, rather than on four equally efficient pass andrejection bands.

FIGS. 8A to 8G represent the spectra of the pulsed clock signal(designated CL) and the four wavelength channels (designated W, X, Y, Z)present at corresponding locations A to G on FIG. 7. As can be seen, thefour channels (W=1554.2 nm, X=1555.6 nm, Y=1557 nm and Z=1558.4 nm) inFIG. 8B are wavelength broadened, as represented in FIG. 8C, by thepulse clock signal (1545 nm) shown in FIG. 8A. FIGS. 8D and 8E show thechannels having been stripped of their cw background component by thefiltering. FIG. 8F represents the four channels after wavelengthconversion by the cw reference signal in the second TWSLA 770, to asingle wavelength OTDM signal. FIG. 8G represents the resultant OTDMsignal at 1545 nm, after the cw background of the reference signal hasbeen filtered away.

FIGS. 9A to 9G represent the time-varying characteristics of the clockand four channels (designated cl, w, x, y, z) which correspond to thesame points A to G on FIG. 7. FIG. 9A represents pulsed clock signal.FIG. 9B represents a data pattern superimposed onto all four channels(all data patterns are the same for each channel for the sake ofclarity). FIG. 9C represents the output of the first TWSLA 745 whichcomprises components of the data pattern and the clock pulse signals.The figure shows an idealised superposition of wave forms where thesignals coincide, which in practice would more closely resemble adisturbance, or glitch, in the data pattern. The output of the filter750 is represented in FIG. 9D. It can be seen in this figure that allbut the perturbed (cross-phase modulated) portions of the wave form arefiltered out, leaving a pulse train corresponding to a RZ representationof the original NRZ signal.

The effect of the dispersive element 760 to disperse each wavelengthchannel by a different amount is shown in FIG. 9E, where the WDM RZpulse train is converted to interleaved pulses w, x, y, z at thedifferent channel wavelengths. This pulse pattern remains the samethroughout the remaining system, wherein only the spectral compositionsof the pulses vary in accordance with FIGS. 8A to 8D.

As the skilled person will appreciate, the systems in FIGS. 1 and 7relate only to possible ways of carrying out the present invention.Particularly, the non-linear elements described could be realised byalternative devices or arrangements of devices other than TWSLAs whichprovide a similar effect, for example NOLMs. In fact, the devicesdescribed in each stage of the invention (the non-linear element, thedispersive element, and the second non-linear element) can each berealised in a variety of ways, without falling outside the bounds of thepresent invention.

Also, all signal levels, signal frequencies, repetition rates and devicebias currents etc, are provided by way of example, and are not essentialfeatures of the present invention.

Furthermore, it will be appreciated by the skilled person that the typeof optical amplification (if any), and the exact positioning of opticalamplification stages in the systems described, depends entirely on thearrangements and types of devices used, and as such is not an essentialaspect of the present invention.

What is claimed is:
 1. Apparatus for processing a wavelength divisionmultiplexed optical signal, comprising:input means to receive a firstoptical signal and a second optical signal, said first optical signalcomprising at least two data channels of different wavelength and saidsecond optical signal being of a single wavelength and comprising astream of pulses, the pulses having a pulse repetition rate at least ashigh as the bit rate of the highest data rate data channel; a non-linearoptical element having input means to receive said first and secondoptical signals and output means to provide a third optical signal, thethird optical signal being representative of a logical AND function ofthe first and second optical signals and comprising pulses each havingwavelength components corresponding to the respective data channels; andmeans to apply a wavelength-dependent delay to each wavelength componentof the third optical signal to provide a fourth optical signal. 2.Apparatus according to claim 1, wherein the delay means comprises adispersive element for providing chromatic dispersion.
 3. Apparatusaccording to claim 2, wherein the dispersive element comprises a lengthof optical fibre member.
 4. Apparatus according to claim 1, furthercomprising a cyclic filter, downstream of the optical element, having apass-band substantially centred at each of the different wavelengthcomponents of the third optical signal.
 5. Apparatus according to claim4, wherein the cyclic filter comprises a birefringent element. 6.Apparatus according to claim 1, further comprising:a second non-linearoptical element to receive the fourth optical signal and a fifth opticalsignal, the fifth optical signal comprising a stream of pulses having asingle wavelength and having a pulse repetition rate at least equal tothe pulse repetition rate of the second optical signal multiplied by thenumber of data channels; and output means to provide a sixth opticalsignal, the sixth optical signal comprising optical pulses of a singlewavelength representative of a logical AND function of the fourth andfifth optical signals.
 7. An apparatus according to claim 1, wherein theor at least one of the non-linear optical elements comprises asemiconductor laser amplifier.
 8. An apparatus according to claim 1,wherein the or at least one of the non-linear optical elements formspart of an optical loop mirror.
 9. An apparatus according to claim 1,wherein the or at least one of the non-linear optical elements comprisesan electro-optic modulator having an electrical clock input means. 10.An optical communications system comprising a first optical fibretransmission path for carrying a wavelength division multiplexed opticalsignal, a second optical fibre transmission path for carrying a timedivision multiplexed optical signal and means connecting the first andsecond transmission paths for converting a wavelength divisionmultiplexed signal into a time division multiplexed signal, said meansfor converting comprising apparatus according to claim
 1. 11. An opticalprocessing system for converting an input optical data signal comprisinga plurality a discrete wavelength square wave data channels to an outputoptical data signal comprising a single wavelength time divisionmultiplexed pulsed signal, said system comprising:a first non-linearoptical device arranged to receive the input signal and a first clocksignal and to provide an output optical pulsed signal representation ofthe original square wave signal, said input signal comprising aplurality of discrete wavelength square wave data channels having arelatively low optical power and said clock signal comprising a pulsetrain having a frequency at least as high as the highest data rate datachannel and a relatively high optical power; means to temporallydisperse the different wavelength components of the output opticalsignal by respective pre-determined amounts to provide awavelength-interleaved output optical pulsed signal; and a secondnon-linear optical device arranged to receive the resultant outputoptical pulsed signal and a second clock signal, and to provide asingle-wavelength output optical pulsed signal, said input signal havinga relatively low optical power and said clock signal comprising a pulsetrain having a frequency substantially equal to that of the pulsedsignal and a relatively high optical power.
 12. A method for processinga wavelength division multiplexed optical signal, comprising the stepsof:introducing a first optical signal and a second optical signal, intoa non-linear optical element, said first optical signal comprising atleast two data channels of different wavelength and said second opticalsignal being of a single wavelength and comprising a stream of pulses,the pulses having a pulse repetition rate at least as high as the bitrate of the highest data rate data channel, the optical processing meansbeing arranged to provide a pulse stream representative of a logical ANDfunction of the first and second optical signals, where each pulseincludes wavelength components corresponding to the respective datachannels; and applying a wavelength-dependent delay to the third signalto provide a fourth, delayed, optical signal.
 13. A method according toclaim 12, wherein the first input signal comprises a plurality ofdiscrete wavelength square wave data channels.
 14. A method according toclaim 12, further comprising the step of introducing the fourth and afifth optical signal into a second non-linear optical element to providea sixth optical signal, the fifth optical signal comprising a stream ofpulses having a single wavelength and having a pulse repetition rate atleast equal to the pulse repetition rate of the second optical signalmultiplied by the number of data channels, the sixth optical signalcomprising optical pulses of a single wavelength representative of alogical AND function of the fourth and fifth optical signals.
 15. Amethod of producing a multiple channel pulsed data signal from amultiple channel square wave data signal, said method comprising thesteps of:introducing a multiple channel square wave data signal into anon-linear optical device; introducing a pulsed clock signal having arepetition frequency at least as high as the highest data rate squarewave data channel into the device; and arranging the respective signalpower levels to provide an output data signal including perturbed datasignal portions resulting from cross phase modulation of the data andclock signals, said perturbed data signal portions corresponding to apulsed signal representation of the square wave data channels.